In the general field of photography, wherever a still or movie camera is used there is always a need to measure the light value. For popular photography including video camcorders and even professional camera work in the field such as news coverage, the light meters used are almost always of the reflective type which senses the light, reflected back from the subject, and in many cameras some or all of the camera settings are then made automatically. This mode is popular because of its convenience since the light meter is typically built into the camera, however since it depends on a single light reading which is an average of various reflectance value highlights in the subject as well as in the background, it is inherently critical regarding its directivity pattern and is necessarily a compromise, subject to errors and unpredictable results particularly in the common condition where there are large variations in the light throughout the field of view.
Cinematography requires detailed accurate measurement and close control of lighting due to the high standards sought and the limitations of photographic film, consequently light meters used in cinematography are typically of the incident type rather than the more common reflected light type.
In the best practice of professional cinematography, before commencing filming, the scene is in effect "mapped" by taking number of incident light readings at various points in the field of view including both the subject and the surrounding background. Before filming, the cinematographer, having determined the field of view and set up the lighting initially, carries a hand-held incident light meter while moving around in the vicinity of the subject where he measures the incident light at several points in the field of view, usually seeking points of maximum or minimum light level. These readings are analyzed, and may be used initially to adjust and improve the lighting and finally to determine the correct camera settings. Typically a required f-stop setting is calculated, based on well-known photographic formulae, from the combination of light reading, film speed, shutter speed, shutter angle, and the attenuation of any lens filters.
In incident light meters the light sensitive element is typically placed behind a translucent diffuser to provide a relatively broad directivity pattern in comparison to the reflective type which is typically as directional as the camera lens angle.